Pulsed electric dichroism studies of the amylose tri-iodide complex

Abstract

The anisotropic optical absorption of macromolecules can be studied, when the molecules are in dilute solution, by subjecting the system to pulsed electric fields. Measurements were made on solutions of six amylose samples of varying molecular weights (M) in the range 1.4 × 10⁵ to 1 × 10⁶, after the biopolymer had been complexed with iodine. The rates of molecular orientation enabled values of τ, the rotary relaxation time, to be obtained. When analysed, these indicated molecular flexibility. The “weakly-bending rod” model appeared to satisfy the experimental data, although a single value of the persistence length (q) did not describe the behaviour. As found in previous studies on other helical polymers, q varied continuously with M. The electrical properties and the degree of orientation in the field indicated that only induced and not permanent dipole moments were present. These induced dipole moments were analysed in terms of an anisotropy of the molecular polarisability. This was consistent with the model of a flexible helix. A value of 28 × 10⁻²⁴ F m was evaluated for the excess polarisability difference per helical pitch. Values of the molecular extinction coefficients for light of wavelength 625 nm when polarised parallel to and perpendicular to the individual helix axes, indicated that the relevant electronic transition is not confined to the long helical axis as had been previously assumed.